U.S. Pat. No. 4,686,940 describes a device for cleaning waste gases from heating systems, in particular heating systems for single or multiple family houses. On this cleaning device the waste gas from the boiler is fed through a chamber in which a mist or rain of a liquid is generated. Periodically, controlled by a control device, milk of lime is added as neutralizing agent to the water in the container. The feeding of neutralizing agent occurs by hydraulic pressure. This is controlled by a magnet valve. This causes water to flow into the neutralizing agent storage means so that neutralizing agent is fed over a pipe into the container in which the cleaning of the waste gas takes place. Because the suspension of lime in water has the tendency to solidify in time, clogging may occur so that no neutralizing agent is anymore transported. If the acid condensates are not anymore neutralized they can cause corrosion and thus destruction of the waste gas cleaning device.
It would seem advisable to use pulverous or granular neutralizing agents instead of liquid neutralizing agents or suspensions of solid neutralizing agents in water. For example, caustic lime could be used as neutralizing agent. However, caustic lime and also some other pulverized and granular neutralizing agents are relatively dangerous substances which can cause cauterization or burns. A special danger occurs when such substances enter the eyes. In addition, the feeding of pulverized substances is considerably more difficult than feeding of fluid substances. Fluid substances can be easily fed by means of valves or pumps. This may be the reason why up to the present time pulverous or granular neutralizing agents have not found application in waste gas cleaning apparatus of small or medium size.
There exist feeding devices for feeding pulverous or granular material from a container to a consuming device by means of an axial feeding member, for example a screw. However, pulverous material has a tendency to stick or clump together. Accordingly, screw feeding devices may turn empty, that is may not feed material, if not additionally vibrators are provided to cause a steady gliding down of material towards the feed screw. But sometime vibrators do not have the desired effect. They also generate undesirable noise.